Circuit protector

ABSTRACT

An electrical circuit and means for protecting the high voltage supply of a microwave oven against faults, shorts or failures. Any failure in, for example, the magnetron energy generator, voltage rectifiers, capacitors or other components in the electrical circuit forming the secondary winding loop coupled to a high voltage step-up transformer, particularly, of the ferroresonant saturable core type results in the actuating of thermally controlled deenergizing means to decouple the oven from the line voltage source thereby preventing damage and/or unsafe operation.

United States Patent 11 1 Peterson 1 1 Feb. 20, 1973 1 CIRCUIT PROTECTOR[75] Inventor: Donald E. Peterson, Iowa City, Iowa [73] Assignee: AmanaRefrigeration, Inc., Amana,

Iowa

22 Filed: March 30,1972

21 Appl.No.: 239,480

[52] US. Cl ..3l7/40 A, 219/481, 219/517, 328/259 [51] Int. Cl. ..H02h5/04 [58] Field of Search ..328/259, 262; 317/40 A; 219/481, 517

{56] References Cited UNITED STATES PATENTS 1,968,198 A 7/1934 Gibbs..328/259 3,385,956 5/1968 Ohara et a] ..2l9/48l Fcinberg ..328/262Tingley ..328/262 X Primary Examiner-James D. Trammell Attorney-HaroldA. Murphy et a].

[57] ABSTRACT An electrical circuit and means for protecting the highvoltage supply of a microwave oven against faults, shorts or failures.Any failure in, for example, the magnetron energy generator, voltagerectifiers, capacitors or other components in the electrical circuitforming the secondary winding loop coupled to a high voltage step-uptransformer, particularly, of the ferroresonant saturable core typeresults in the actuating of thermally controlled deenergizing means todecouple the oven from the line voltage source thereby preventing damageand/or unsafe operation.

6 Claims, 5 Drawing Figures MICROWAVE HEATING APPARATUS PATENTED FEB 2 0I975 sum 2 er 2 cmcurr PROTECTOR BACKGROUND OF THE INVENTION ponentfailure which causes an increase in current in any portion of thecircuit.

2. Description of Prior Art An energy generator widely used in microwavecooking is the magnetron. Such devices are energized by domestic lowfrequency, low voltage sources and high voltage supplies capable ofgenerating rectified DC voltages at levels of, for example, 4,000 to6,000 volts. In the magnetron electric and magnetic fields extend withinan interaction region defined between a cathode and circumferentiallydisposed anode cavity resonators. The electrons are accelerated towardsthe cavity resonators and rotate in a substantially helical path to forma rotating spoke-like 'space charge and interact in energy exchangingrelationship with the electric fields to generate microwave energy. Theterm microwave" is defined as electromagnetic energy having wavelengthsin the order of approximately 30 centimeters to l millimeter andfrequencies in excess of 300 MHz. The starting voltages are appliedbetween the anode and cathode by circuits coupled to the secondarywinding of a high voltage step-up transformer. The magnetron isinherently a unidirectional device since oscillations are generated onlywhen the anode is positive relative to the cathode on alternatehalf-cycles. Rectifying means are, therefore, employed in combinationwith the high voltage transformers to provide for continuous operationof the energy generator in microwave cooking. Full-wave or half-wavevoltage doubler circuits are customarily employed for suchrectification. Such circuits include numerous capacitors andsemiconductor diodes.

A high voltage transformer which has evolved in the art to provide forcontrolled constant current in the pulses to energize the magnetron andthereby eliminate the effects of line voltage fluctuations is disclosedin US. Letters Pat. No. 3,396,342, issued Aug. 6, 1968, to A.E.Feinberg. Such transformers have the primary and secondary windingselectrically isolated from one another and are coupled in a high leakagereactance operating relationship by means of a ferroresonant saturablecore. Voltage variations in the primary wind ing loop, therefore, havevery little effect on the magnetron current. Energy storage means areconnected in the secondary winding loop and return paths for the currentthrough such energy storage means on alternate half-cycles are provided.In the operation of such saturable core transformers with full-wave orhalf wave voltage doubler circuits, a failure due to a fault in theenergy generator or any of the circuit components normally causes anincrease in current in the secondary loop. Due to the isolation of thesecondary and primary windings such an increase in the current does notresult in a corresponding increase in the current in the primary windingloop. It is customary in electrical circuits for operating microwaveovens to provide electrical circuit breakers in the line voltage circuitto handle component failures which could result in damage or unsafeoperation. In saturable core transformers, however,

there is an absence of any protection from failures in the secondarywinding loop to deenergize the oven. It is necessary, therefore, thatsuitable circuit protectors be provided, particularly, in microwave ovenapparatus to prevent permanent damage to costly magnetrons, high voltagetransformers and the components of the voltage rectification circuits aswell as unsafe operation.

SUMMARY OF THE INVENTION A high voltage supply electrical circuit isprovided for a microwave oven apparatus including a step-up transformerhaving primary and secondary windings. Means are provided in thesecondary winding loop to sense a predetermined rise in the secondarycurrent. In an embodiment a resistor is serially connected in thesecondary winding loop. Integral with the resistor is a thermallycontrolled deenergizing element in series with the primary winding loopand line voltage source.

Any equipment failure such as a shorted magnetron energy generator,inoperative diode rectifier, capacitor, or equivalent component faultwhich results in an increase in the secondary current will thermallyactuate the protection means. Since the power dissipated by the resistorincreases in proportion to the square of the current, a resistor ofrelatively low value, such as 4 ohms and 5 watts, can be employed. Asufficient rise in temperature will result to cause the adjacentthermally controlled element to disintegrate which will deenergize theoverall microwave oven electrical circuit. Serious damage as well asunsafe operation is thereby avoided.

The invention is equally applicable to protect any circuits having highvoltage transformers for such applications as radar, industrialmicrowave processing systems and communication equipment.

BRIEF DESCRIPTION OF THE DRAWINGS Details of an embodiment of theinvention will be readily understood after consideration of thefollowing description and reference to the accompanying drawings,wherein:

FIG. l is a schematic circuit diagram of the illustrative embodiment ofthe invention;

FIG. 2 is an elevation view, partially in section, of the sensing andthermally controlled deenergizing means embodying the invention;

FIG. 3 is an exploded elevation view of the principal components of theembodiment of the invention shown in FIG. 2;

FIG. 4 is a cross-sectional view of an exemplary thermally controlledelement in the closed circuit position; and

FIG. 5 is a cross-sectional view of the element shown in FIG. 4 in thebreak of open circuit position for deenergizing the overall electricalcircuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings,FIG. 1 is a schematic circuit diagram illustrative of the embodiment ofthe invention used in combination with microwave heating apparatus 10.The high voltage supply circuit is coupled to a domestic or industriallow frequency, low voltage source 12 by means of leads l4 and 16 througha manually operated stop-start switch 18. A saturable core transformer20 having a high leakage reactance of the type disclosed in theafore-referenced US. Pat. No. 3,396,342 has primary winding 22 connectedto the line leads 14 and 16. The thermally controlled deenergizingelement 24 of the invention is serially connected in the primary windingcircuit.

Secondary winding 26 which is electrically isolated from the primarywinding has, illustratively, a turns ratio of 40-5021 to provide thehigh voltages to energize the magnetron energy generator 28. The energygenerator is of the well-known type having a cathode 30 generally of theoxide-coated type with a directly heated filament and an anode 32defining a plurality of cavity resonators 34 circumferentially disposedabout the cathode 30. The high frequency microwave energy generated bymagnetron 28 is coupled by an antenna loop member 36 throughconventional waveguide means to the oven enclosure defined by theconductive walls of microwave heating apparatus in a manner well knownin the art. Means may also be provided for distributing the energy, suchas stirrers.

The high voltage rectification means comprise a fullwave voltage doublercircuit 38 coupled to the secondary winding 26. The circuit includessemiconductor diodes 40 and 42, electrically biased in the manner shownin the illustration. Capacitors 44 and 46 are also connected in thecircuit. In operation, when the end of secondary winding 26 connected tolead 48 is poled positive, the current flows through semiconductor diode42 to charge capacitor 46 to the predetermined voltage. Semiconductordiode 40 in this portion of the cycle remains nonconductive.

On the next half-cycle, lead 50 becomes poled positive and the currentflows through diode 40 to charge capacitor 44. In this portion of thecycle diode 42 remains nonconductive. The total rectified voltageapplied between the anode and cathode of the magnetron energy generator28 is the sum of the voltages or approximately twice the voltage appliedacross each capacitor 44 and 46. Other high voltage rectifying means maybe employed in the practice of the invention such as half-wave voltagedoubler circuits, voltage triplers or quadruplers.

In view of the electrical isolation of the secondary winding andaccompanying loop circuit from the primary winding of the transformerany failures in the components, such as a shorted magnetron energygenerator, faulty diodes or capacitors which could result in damage orunsafe operation would not cause an increase in the primary voltagecurrent to trip a conventional circuit breaker or fuse at the source. Afault in the secondary circuit, however, does cause an increase incurrent. It is this current rise which is utilized in the invention bymeans of a sensing element serially connected in the secondary windingloop circuit to actuate the thermally controlled deenergizing element 24in the primary winding circuit.

Referring now to FIGS. 2 and 3, an exemplary embodiment 52 of theinvention, comprises a wirewound resistor 54 having terminals 56 and 58for serially connecting to the secondary winding loop circuit. An increase in the secondary current increases the power dissipated by theresistor 54 in proportion to the square of the current. A source ofthermal energy is thereby provided which is utilized to actuate thethermally controlled element 24. In an exemplary embodiment a wirewoundresistor 54 having a value of 4 ohms and power dissipation of 5 wattswas found to function satisfactorily The resistor circuit normallycarries a current of L0 amperes R.M.S. and is at approximately 8 ,000volts potential.

An exemplary thermally controlled element 62 is integrally mountedadjacent to the resistor and is isolated by means of insulating tape 60.The element 62 is selected to operate with a temperature rise ofapproximately 358 F. A thermal limiter, available under the trade nameMicro Temp, comprises a nonconductive material which rapidly changesfrom a solid to a liquid state at a predetermined temperature to actuatea spring-loaded contact and open an accompanying electrical circuit.lnsulating tape 60 is wound around the element 62 to electricallyisolate it from the secondary winding circuit. Leads 64 and 66 togetherwith sleeve insulators 68 and 70 provide for connecting the thermalelement 62 to the primary circuit through leads 14 and 16. The primarycircuit connected through the thermal element handles a current ofapproximately 20 amperes R.M.S. during normal operation.

Referring now to FIGS. 4 and 5, thermally controlled element 62 isillustrated in greater detail. Such thermal devices which have evolvedin the commercial market are capable of being miniaturized so that in anembodiment for protection of a microwave oven circuit an element havinga length of typically 1% inches can be employed with a wirewoundresistor 54 having an inner diameter of approximately three-sixteenthsof an inch. In FIG. 4 the element is shown in the closed circuitposition. Wire lead 64 is encased in a sealing compound 72 secured atone end of case 74 while lead 66 is disposed at the opposing end and isconductively secured to the case 74. A pellet 78 of a nonconductivematerial which rapidly changes from a solid to a liquid state at apredetermined temperature is disposed in contact with disc member 76 tourge spring 82 against disc member 80. Metallic star contact member 84is in contiguous relationship with disc member and the interior walls ofthe conductive case 74 as well as the inner end of lead 64. A tripspring 86 urges the contact member 84 in the opposite direction and ismaintained in position by ceramic bushing 88 through which the lead 64extends. In normal operation, with the thermal element 62 carrying thenormal primary winding current, lead 64 provides a current path throughstar contact member 84 and the inner walls 740 of the case 74 adjacentpellet 78. The current path is completed through the opposing lead 66.

In FIG. 5 the thermal element 62 is shown in the open circuit positiondeenergizing the primary winding circuit upon the sensing of a fault inthe secondary winding loop by the resistor 54. Upon reaching theillustrative threshold temperature of 358 F the pellet 78 willdisintegrate. The circuit will be broken when lead 64 no longer contactsstar contact member 84 to provide positive protection. Deenergizing ofthe overall microwave oven electrical circuit will prevent seriousequipment damage as well as unsafe operation. It will be noted thatspring 82 has been fully extended with disc member 76 positioned againstthe interior of lead 66.

There is thus disclosed an efficient and positive operating circuitprotector for sensing faults in components coupled to the secondarywinding loop of a high voltage core transformer. A rise in temperaturedue to an increase in secondary current deenergizes the primary windingcircuit. In the operative position the thermal deenergizing element isnot required to carry the high secondary voltages which in the case ofmicrowave oven apparatus can be many thousands of volts. The circuit isunique in that equipment failure results in complete decoupling of thehigh voltage supply circuit from the line voltage source. The oven canbe operated again only after qualified service personnel have replacedthe faulty component and inserted' a new circuit protector.

Numerous modifications, variations or alterations may be practiced bythose skilled in the art. It is intended, therefore, that the foregoingillustrative embodiment and detail description be considered in itsbroadest aspects and not in a limiting sense.

Iclaim:

1. In combination:

a voltage source;

high voltage transformer means having primary and secondary windingswith said primary winding connected to said voltage source;

an electrical circuit connected to said secondary winding to generatehigh output voltages;

a circuit protector including means for sensing a predetermined currentincrease due to circuit faults operatively associated with saidsecondary winding circuit; and

deenergizing circuit means operatively associated with said primarywinding and disposed adjacent to said sensing means;

said deenergizing means being electrically isolated from said sensingmeans.

2. The combination according to claim I wherein said sensing meanscomprise a resistor.

3. The combination according to claim 1 wherein said deenergizing meansare thermally actuated.

4. In combination:

a voltage source;

high voltage transformer means having electrically isolated primary andsecondary windings with said primary winding connected to said source;

an electrical circuit connected to said said secondary winding includingcomponents for voltage storage, rectification and generation of highoutput voltages;

a circuit protector including means for sensing a predetermined currentincrease due to component failure in said secondary winding circuit; and

thermally actuated deenergizing means operatively associated with saidprimary winding and disposed adjacent to said sensing means.

5. In combination:

a voltage source;

an energy generator;

an electrical circuit including high voltage transformer means havingprimary and secondary windings with said primary winding connected tosaid voltage source and a circuit loop including voltage rectificationmeans connected to said secondary winding to generate high voltages tooperate said energy generator; a circuit protector including means forsensing a predetermined current increase in said secondary windingcircuit loop; and

deenergizing circuit means disposed in thermal con tact with saidsensing means;

said deenergizing circuit means being electrically isolated from saidsensing means.

6. Microwave heating apparatus comprising:

an oven enclosure;

an electromagnetic microwave energy generator;

means for radiating said microwave energy within said enclosure;

electrical circuit means for energizing said generator including avoltage source, transformer means having primary and secondary windingsand high voltage rectification means; v

a circuit protector having means for sensing a predetermined currentincrease due to operative failure in said high voltage rectificationmeans and energy generator; and

deenergizing circuit means thermally associated with said sensing means;

said deenergizing means being electrically isolated from said sensingmeans.

1. In combination: a voltage source; high voltage transformer means having primary and secondary windings with said primary winding connected to said voltage source; an electrical circuit connected to said secondary winding to generate high output voltages; a circuit protector including means for sensing a predetermined current increase due to circuit faults operatively associated with said secondary winding circuit; and deenergizing circuit means operatively associated with said primary winding and disposed adjacent to said sensing means; said deenergizing means being electrically isolated from said sensing means.
 1. In combination: a voltage source; high voltage transformer means having primary and secondary windings with said primary winding connected to said voltage source; an electrical circuit connected to said secondary winding to generate high output voltages; a circuit protector including means for sensing a predetermined current increase due to circuit faults operatively associated with said secondary winding circuit; and deenergizing circuit means operatively associated with said primary winding and disposed adjacent to said sensing means; said deenergizing means being electrically isolated from said sensing means.
 2. The combinAtion according to claim 1 wherein said sensing means comprise a resistor.
 3. The combination according to claim 1 wherein said deenergizing means are thermally actuated.
 4. In combination: a voltage source; high voltage transformer means having electrically isolated primary and secondary windings with said primary winding connected to said source; an electrical circuit connected to said said secondary winding including components for voltage storage, rectification and generation of high output voltages; a circuit protector including means for sensing a predetermined current increase due to component failure in said secondary winding circuit; and thermally actuated deenergizing means operatively associated with said primary winding and disposed adjacent to said sensing means.
 5. In combination: a voltage source; an energy generator; an electrical circuit including high voltage transformer means having primary and secondary windings with said primary winding connected to said voltage source and a circuit loop including voltage rectification means connected to said secondary winding to generate high voltages to operate said energy generator; a circuit protector including means for sensing a predetermined current increase in said secondary winding circuit loop; and deenergizing circuit means disposed in thermal contact with said sensing means; said deenergizing circuit means being electrically isolated from said sensing means. 